The invention relates to devices for counting events from which two signals in time coincidence may be derived. It is of particular interest for counting disintegrations in a sample labelled with one or more radioactive tracers, inter alia by liquid scintillation spectrometry.
The determination of the activity of samples labelled with beta emitters by liquid scintillation spectrometry has numerous advantages; the counting yield is high even for low-energy beta emitters such as .sup.3 H and .sup.14 C; background noise is relatively low; and the samples are easy to prepare.
Liquid scintillation spectrometry, however, has some shortcomings. The counting efficiency is lowered by light attenuation phenomena conventionally called "quenching." This shortcoming has been overcome by using standardization methods which have now been well developed.
Another problem which occurs in the certain cases is related to background noise due to chemical interactions in the sample or between the sample and the other components (e.g. a solvent, solubilizing agent, wetting agent, or scintillator) in the liquid solution in which the sample is present. The interactions result in the emission of single photons and produce a background noise in addition to the noise from other origins. That additional background noise may be quite variable. It is known under the general term "chemi-luminescence. "
This interfering luminescence results in the appearance of single photons, whereas the events to be counted are represented by showers of photons resulting from an energy exchange between the radiation and the scintillator. A large fraction of the signals due to chemi-luminescence may be eliminated by pulse height analysis, and by the use of two photomultipliers associated with a coincidence circuit in modern spectrometers. The coincidence circuit, which is provided to eliminate the effects of thermionic emission in each photomultiplier, reduces the background noise by a ratio of the order of 10 -.sup.5 for typical resolving times .sup.-of the coincidence circuit. In many cases, however, this ratio is insufficient. The emission rate of single photons by chemi-luminescence results in a background noise which is not negligible compared with the true activity of the test sample, and adds an unknown and significant contribution.
Various methods have already been suggested for reducing the effects on counting on phenomena which result in the emission of a high rate of single photons producing random coincidencies, such as chemi-luminescence.
One such method, which was described in "Nuclear Instruments and Methods"73 (1969), pages 67 -76, includes the step of recording the number of time coincidences detected by a circuit, the first input of which directly receives signals coming from a second detector via a delay line.
If the detectors are assumed to be photomultipliers, there is no time correlaion between spontaneous emissions of single photo-electrons by the photomultipliers and photon emission by chemi-luminescence is of random nature; the rate of "delayed"coincidences will be equal to the rate of random coincidences to which a "prompt"or "direct"coincidence circuit is subject during the same time interval, provided that the two coincidence circuits have the same resolving times, and subject to statistical errors. On the other hand, the delayed coincidence circuit is normally insensitive to true coincidences recorded by the prompt coincidence circuit.
This solution is not entirely satisfactory. If, during a single time interval, a count is made of :
all the prompt and delayed coincidences and PA1 the delayed coincidence only (i.e. the random coincidences),
then, two high-capacity counters are needed. The device does not give information in real time. If the pulse rate is very high and variable, the correction given by statistical subtraction is not complete. Finally, present day systems of that type cannot provide pulse height analysis in real time.
In a simpler method, several countings of the acitivity rate of a sample are successively carried out. If it is found that the rate decreases between successive measurements, it is assumed that chemi-luminescence is present and counting of the sample is delayed until the decrease is not appreciable. In some cases, the decrease continues for a very long time, so that the sample has to be rejected without counting. It is never possible to make an immediate determination of the true activity of a sample exhibiting chemi-luminescence.